Reverts change I65ef5886af59ec85841734f03bfac781194f780a which
introduced a breaking change for submodule projects inside a docs
project. Docs builds have a separate script which handles fetching
patches from Gerrit so should not be setting the refspec in the SCM
plugin.
Change-Id: I657a473a274b2831ad1470bb4895a2c8bcf3c92f
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>